Magma Genesis Beneath Continents

University Of California-Los Angeles, Los Angeles CA

Investigators

Abstract

Bryce EAR-0207939 Continental basalts provide the broadest insights into the nature and origin of the continental lithosphere but, because they may represent a mixture of contributions acquired during melt formation, transport from the source region, evolution in a magma chamber, and eruption, the chemical identities and the relative roles of these processes are topics of debate. Isotopic and chemical studies of melt inclusions in phenocrysts of mafic lavas provide a means to examine and quantify contributions from these processes before exotic melts have been mixed away during melt aggregation and magma storage. The focus of the proposed work is to investigate these contributions in the 0-8 Ma alkali basalts of the Cima volcanic field of eastern California. Mantle xenoliths contained within the lavas demonstrate that the mantle at shallow levels beneath Cima is compositionally heterogeneous and that the lavas had a limited residence time in crustal chambers. Accordingly, the large variations in Pb isotopic compositions observed in magmas of the Cima suite most likely reflect signatures acquired during their formation and transport through the mantle, rather than from assimilation of continental crust. The proposed study will combine in situ techniques (e.g., electron and ion microprobe) to characterize the major, trace, and Pb isotopic compositions of melt and mineral inclusions in olivine and other phases from mafic (MgO > 6 wt. %) Cima lavas. Our results will address whether the compositional and Pb isotopic variety of melt inclusions trapped in continental lavas is even greater than that observed in the suite of lavas that contain them, from which we can better delimit the nature and origin of compositional domains in the mantle at levels deeper than those sampled by xenoliths. The results from this project can be combined with ongoing studies of the isotopic and compositional variability of melt inclusions trapped in oceanic basaltic phenocrysts to improve our understanding of the processes significant in the generation of basaltic magmas.